Actuatable passenger restraint systems for vehicles are well known in the art. Such systems include an inflatable air bag mounted within the passenger compartment of the vehicle Each air bag in the vehicle has an associated, electrically actuatable ignitor, referred to as a squib. Such systems further include an inertia sensing device for measuring the deceleration of the vehicle When the inertia sensing device indicates that the vehicle is decelerating a certain amount, an electric current of sufficient magnitude and duration is passed through the squib to ignite the squib which results in inflation of the air bag.
Many known inertia sensing devices used in actuatable passenger restraint systems are mechanical in nature Such devices are typically mounted to the vehicle frame and include a pair of mechanically actuatable switch contacts and a resiliently biased weight The weight is arranged such that when the vehicle is decelerated, the weight physically moves relative to its mounting The greater the deceleration, the more the weight moves against the bias force. The switch contacts are mounted relative to the biased weight such that, when the weight moves a predetermined distance, the weight moves over the switch contacts causing them to close. The switch contacts, when closed, connect a squib to a source of electrical energy sufficient to ignite the squib.
Still other known actuatable passenger restraint systems for vehicles include an electrical transducer or accelerometer for sensing vehicle deceleration. Such systems include a monitoring or evaluation circuit connected to the output of the transducer. The monitoring circuit, upon determining the signal from the transducer indicates a probable crash is occurring, actuates an electrical switch so as to connect electrical energy to the squib. U.S. Pat. No. 3,870,894 to Brede, et al., ("the '894 patent") discloses such an actuatable passenger restraint system.
The '894 patent teaches a system which includes an accelerometer, an evaluation circuit connected to the accelerometer, and an ignition circuit or squib connected to an output of the evaluation circuit. The accelerometer includes a piezoelectric transducer that provides an electrical output signal having a value indicative of the vehicle deceleration. The evaluation circuit includes an integrator electrically coupled to the output of the accelerometer through an amplifier. The output of the integrator is an electrical signal having a value which is functionally related to the integral of the deceleration signal. When the output of the integrator reaches a predetermined value, the ignition circuit of the air bag is fired.
The integrator used in the '894 patent is a common resistor/capacitor configuration known in the art. The output of the integrator ramps up from electrical ground toward the supply voltage at a rate that is functionally related to the value of the input signal. A trigger circuit is connected to the output of the integrator. When the output of the integrator reaches a predetermined value, the trigger circuit actuates a time delay circuit. The time delay circuit begins to time out a predetermined time period. After the time period is timed out, the air bag ignitor is energized.
The integrator in the '894 patent, in combination with the trigger circuit, provides an indication of the change in velocity of the vehicle. The faster the vehicle deceleration, the faster the trigger circuit will provide the signal to initiate the timing out of the time delay circuit. Therefore, when the vehicle's deceleration is sufficient to result in the accelerometer transducer providing an electrical signal, and when the change in vehicle speed exceeds the predetermined value set by the integrator/trigger circuit combination, the air bag is inflated.
It has been discovered that it is not desirable to inflate the vehicle air bag under all types of rapid decelerations or crashes to which the vehicle is subjected. It is not desirable, for example, to inflate the air bag during a low speed, "soft crash." The determination as to what occurrences fall within the definition of "soft crash" is dependent upon various factors related to the type of vehicle. If, for example, a large vehicle traveling eight miles per hour hits a parked vehicle, such a crash would be considered a "soft crash" that would not require the air bag to inflate to protect the vehicle passengers. The vehicle seat belts alone would be sufficient to provide passenger safety. During such a "soft crash," a typical accelerometer would provide an output signal indicating a rapid deceleration is occurring. In an actuatable passenger restraint system made in accordance with the '894 patent, the air bag would be inflated as soon as the predetermined speed differential occurred and the time delay circuit timed out.
It is desirable to have an actuatable restraint system that can distinguish a "hard crash" from a "soft crash" based upon the accelerometer output signal, and in response thereto, only inflate the air bag when actually needed to protect the passengers in the vehicle.